A magnetic induction flowmeter is preferably used as a flowmeter for liquids, pulps and pastes which have a specific minimum electric conductivity. This type of flowmeter is distinguished by quite accurate measured results, no pressure loss in the pipeline system being caused by the measurement. In addition, magnetic induction flowmeters have no parts which move or project into the measuring tube and which are especially subject to wear. The area of use of the flowmeters of interest here extends primarily to applications in the chemical industry, pharmaceuticals and the cosmetic industry, and also the communal water and effluent business as well as the foodstuffs industry.
Faraday's induction law forms the physical basis of the measuring method of a magnetic induction flowmeter. This natural law states that a voltage is induced in a conductor moving in a magnetic field. During the utilization of this natural law for measurement, the electrically conductive medium flows through a measuring tube in which a magnetic field is generated at right angles to the flow direction. The voltage induced in the medium is picked up by an electrode arrangement. Since the measured voltage obtained in this way is proportional to the average flow velocity of the flowing medium, the volume flow of the medium can be determined from it. In addition, by taking account of the density of the flowing medium, its mass flow can also be determined.
WO 93/09403 discloses a generic magnetic induction flowmeter. Its electrode arrangement, comprising two measuring electrodes let into the measuring tube opposite each other, interacts with a magnetic coil arrangement, which generates the required magnetic field at right angles to the flow direction in the measuring tube. Within this magnetic field, each volume element of the flowing medium moving through the magnetic field, with the field strength present in this volume element, supplies a contribution to the measuring voltage, picked up by the measuring electrodes. The measuring voltage is supplied to the input of evaluation electronics which are connected downstream and which carry out the subsequent measured value processing.
The measuring tube of the magnetic induction flowmeter, which is composed of metal, is electrically conductive and therefore lined with an insulating layer composed of a non-conductive material, in order that the measuring principle functions. This ensures that the flow medium does not come into contact with the electrically conductive measuring tube, which is normally at ground potential, but comes into contact with the insulating layer from which the measuring electrodes that make contact with the substance to be measured protrude. Here, the insulating layer is constructed in the manner of what is known as a liner, that is to say a body which is tubular in the basic state, which is drawn into the measuring tube and is then bent over outward in the flanged region in order to form a rim. The liner is fixed to the metallic measuring tube via the outwardly bent rim. For this purpose, the rim is clamped in via an external annular end plate by means of a screw fixing. Thus, the annular end plate forms the sealing region for the corresponding flange of the pipeline adjacent thereto. The liner is also fixed to the measuring tube via the measuring electrodes in the central region of the latter.
One disadvantage of this solution for fixing the electric insulating layer to the measuring tube is that this is quite complicated to implement in terms of fabrication. It requires additional components, in particular the annular end plate, in order to implement this clamping solution. Moreover, a plurality of screw fixings are needed in the peripheral region of the end flange in order to ensure secure retention of the liner on the measuring tube, which results in corresponding expenditure on mounting.
In addition, the general prior art discloses a fixing solution for the insulating layer of a magnetic induction flowmeter to the measuring tube which, as opposed to the prior art described above, is formed in the manner of a non-detachable connection between insulating layer and measuring tube. In order to ensure secure retention of the insulating layer on the measuring tube, in this case grooves are introduced in the region of the sealing surface of the measuring tube flange for the purpose of connecting the insulating layer to the measuring tube in a form-fitting manner. The grooves are primarily designed in the form of dovetail grooves in order to ensure a form fit. The insulating layer is normally applied to the inner surface and the flanged region of the measuring tube by injection molding. The material used is a thermoplastic which, during the injection molding, gets into the grooves on the side of the measuring tube, so that the result is a secure, non-detachable connection between insulating layer and measuring tube. Since this technical solution assumes the introduction of grooves into the measuring tube, fabrication problems occur in particular when the measuring tube is to consist of a stainless steel, that is to say a non-rusting steel, as is required in the foodstuffs industry, for example. This is because it is quite complicated to machine stainless steel by removing material.